Normally, an optical disk, such as a DVD and a CD, has a light transmitting layer on a recording layer in which information is recorded. Information is recorded in or reproduced from the recording layer by irradiating light onto the recording layer via the light transmitting layer. In a case where the light transmitting layer has an error in thickness and has a difference from a specified value in this instance, this error gives rise to spherical aberration. There is known an optical disk recording/reproducing device configured to correct such spherical aberration, in which a collimator lens formed to be movable is mounted on the optical head, so that the collimator lens is moved to cancel out the spherical aberration according to the thickness of the light transmitting layer (for example, see Patent Document 1).
FIG. 9 is a perspective view showing the configuration of a collimator lens drive mechanism employed in a conventional optical disk recording/reproducing device. A lens drive mechanism 160 shown in FIG. 9 is configured to move a collimator lens so that spherical aberration is cancelled out according to the thickness of the light transmitting layer of an optical disk. It includes a reference shaft 161 and a sub-reference shaft 162 both disposed parallel to the optical axis and a collimator lens holder 163 supported on these reference shafts 161 and 162.
The reference shaft 161 and the sub-reference shaft 162 are immovably attached to a fixing portion of the optical head. The collimator lens holder 163 is supported on these reference shafts 161 and 162 to be movable by sliding in the optical axis direction. The collimator lens operated to move so as to cancel out the spherical aberration is mounted on the collimator lens holder 163. In other words, the lens drive mechanism 160 moves the collimator lens mounted on the collimator lens holder 163 back and forth and parallel to the optical axis direction by moving the collimator lens holder 163 back and forth along the reference shaft 161 and the sub-reference shaft 162.
The lens drive mechanism 160 includes, as drive mechanisms that move the collimator lens holder 163 back and forth along the reference shaft 161 and the sub-reference shaft 162, a DC motor 164 that serves as a drive source to move the collimator lens holder 163 and a gear mechanism 165 that converts rotations of the DC motor 164 into parallel movements along the optical axis direction and transmits the parallel movements to the collimator lens holder 163. Rotations of the DC motor 164 are converted into parallel movements along the optical axis direction by the gear mechanism 165 so as to move the collimator lens holder 163. The collimator lens is thus moved to cancel out the spherical aberration according to the thickness of the light transmitting layer of an optical disk.
The gear mechanism 165 includes a rack 166 attached to the collimator lens holder 163, a first gear 167 attached to the rotation shaft of the DC motor 164 for transmitting a rotational force of the DC motor 164, a second gear 168 to convert rotations of the DC motor 164 into parallel movements along the optical axis direction, a third gear 169 to transmit a drive force that has been converted into the parallel movements along the optical axis direction by the second gear 168 to the rack 166.
The rack 166 to which the drive force is transmitted from the third gear 169 is of a double structure in which two racks 166a and 166b are superimposed and the racks 166a and 166b are coupled with a spring 171 in order to eliminate a backlash between the rack 166 and the third gear 169.
When the collimator lens is moved using the lens drive mechanism 160, the DC motor 164 is rotated. The first gear 167 consequently starts to rotate. Rotations of the first gear 167 are transmitted to the second gear 168 and converted into parallel movements along the optical axis direction. The drive force converted into the parallel movements along the optical axis direction by the second gear 168 is transmitted to the rack 166 via the third gear 169.
The rack 166 is attached to the collimator lens holder 163 and the collimator lens holder 163 is supported on the reference shafts 161 and 162 to be movable by sliding in the optical axis direction. Accordingly, the collimator lens holder 163 is moved in the optical axis direction by the drive force transmitted to the rack 166 via the third gear 169. The collimator lens mounted on the collimator lens holder 163 is thus moved in the optical axis direction.
The lens drive mechanism 160 configured as above is able to move the collimator lens at some degree of accuracy and is therefore able to correct the spherical aberration resulting from a variance of the light transmitting layer in thickness.
In the lens drive mechanism 160 described above, however, rotational motions of the DC motor 164 within a perpendicular plane is converted to rotational motions within a horizontal plane using the first gear 167, the second gear 168, and the third gear 169 first and thence the rotational motions within a horizontal plane is converted to linear motions using the third gear 169 and the rack 166. This requires a large number of members as motion converting members. In a case where a large number of members are used as above, work errors among the respective members or the like are accumulated, which gives rise to an error at the moving position of the collimator lens holder 163, that is, the collimator lens.
Also, the collimator lens holder 163 is driven using the both slopes of each tooth formed in the respective gears 167 through 169 and the rack 166. Accordingly, in a case where the collimator lens holder 163, that is, the collimator lens, is allowed to reciprocate, one of the slopes of the tooth abuts on the gear when it is moved in a forward direction and the other slope abuts thereon when it is moved in a backward direction. This, in addition to the error described above, gives rise to an error caused by a backlash when the movement directions are switched. It therefore becomes impossible to allow the collimator lens holder 163, that is, the collimator lens, to reciprocate at a high degree of accuracy.
Further, in a case where the third gear 169 and the rack 166 abut on each other using the both slopes of the teeth as described above, it is necessary to increase a pushing force of the spring 171 in order to strengthen the engagement between the third gear 169 and the rack 166. Such a large pushing force becomes a large load for the third gear 169, that is, the DC motor 164. Accordingly, it also becomes impossible to allow the collimator leans holder 163, that is, the collimator lens, to reciprocate at a high degree of accuracy with a low drive force.
Patent Document 1: JP-A-11-259906